The presence of low-friction interfaces allowing shearing motion between tissue surfaces is critical for the normal function of many structures in the body. Biological evolution has provided several solutions for this requirement. Apposed serosal surfaces in the pleura, pericardium, and peritoneum provide a slip interface needed for the normal function of the lungs, heart, and intra-abdominal organs, respectively. Bursal structures allow the shearing motions necessary for function of tendons and parts of the musculoskeletal system. Areolar adipose tissue surrounding blood vessels, muscles, and the capsules of organs provides for smaller degrees of shear motion, allowing relative movement between structures during gross body motion and breathing.
The loss of functional shear interfaces, such as due to scarring or tumor invasion, can lead to serious consequences. For instance, the development of adhesions between the visceral and parietal peritoneum in the abdomen, typically due to scarring following surgery, can lead to functional impairment, intermittent bowel obstruction or acute conditions with catastrophic ischemic consequences. Adhesions in peritendinous tissues can cause serious impairment in extremity and hand function. The loss of normal areolar tissue around the carotid arteries following radiotherapy, and its subsequent replacement with fibrosis, subjects these structures to increased mechanical stress during normal body motion, which is thought to be instrumental in the accelerated development of atherosclerotic changes in these patients.
Conventional imaging techniques such as MRI and CT can depict the gross morphology of tissues at structural shear interfaces and may demonstrate focal thickening or other changes that are likely to be associated with loss of slip functionality. However, they do not directly assess the slip functionality at tissue interfaces.
An imaging technique called MR Elastography (MRE) that can measure the elasticity of tissues has been introduced as disclosed in U.S. Pat. No. 5,592,085. Oscillatory stresses are applied to tissues-of-interest and tissue displacement due to the resulting propagation of shear waves is imaged by encoding the motion into the phase of the MR signals. From images reconstructed from these MR signals, the mechanical properties of the subject can be determined. In many applications, the production of shear waves in the tissues is merely a matter of physically vibrating the surface of the subject with an electromechanical device such as that disclosed in U.S. Pat. No. 5,952,828 or by an acoustic driver such as that described in co-pending U.S. patent application Ser. No. 10/860,174 filed on Jun. 3, 2004.
While the above-discussed method allows the mechanical properties of tissues to be imaged, it does not currently allow the degree of connectivity at tissue slip interfaces to be analyzed. Specifically, it cannot distinguish between functional shearing interfaces and damaged shearing interfaces in which shearing motion between two opposing tissues is at least partially impeded. There is evidence that MRE-based techniques may be used to assess the weldedness of tissue interfaces, as proposed by Papazoglou, et al., in “Horizontal shear wave scattering from a nonwelded interface observed by magnetic resonance elastography.” Phys. Med. Biol. 2007; 52:675-684. However, this is an indirect method that uses a complicated mathematical algorithm to model the scattering of shear waves at tissue interfaces.
It would therefore be desirable to have a simple and direct method for sensitively assessing mechanical shear connectivity across tissue interfaces, which would allow for the identification of damaged tissue interfaces.